JP7346653B2 - Radiation shielding structure for electronic devices - Google Patents

Description

Embodiments of the present invention relate to radiation shielding structures for electronic devices.

In recent years, in the space industry, businesses using small satellites have been launched by venture companies. In this type of business, electronic devices (including electronic equipment) used in small satellites are not commercially available on COTS (Commercial Off-The-Shelf) because it takes time and cost to develop them for use in space. The use of commercially available products and ready-made products is increasing.

However, commercially available products and ready-made products are not designed to withstand space environments, and their resistance to radiation such as gamma rays cannot be guaranteed. For this reason, a process is established in advance to expose electronic devices to radiation using a radiation irradiation facility to screen for the degree of performance deterioration and the occurrence of failures.

Furthermore, in the design of on-board satellites, heavy metals that have a shielding effect (e.g., tungsten, lead) are used to provide housings and shielding structures for housing electronic devices, circuit boards, etc.

Special Publication No. 2006-518112 Special Publication No. 2007-531981 Japanese Patent Application Publication No. 2002-166899

3rd Space Seminar "Space Environment for Spacecraft Design", Japan Aerospace Exploration Agency, Haruhisa Matsumoto, May 8, 2015, Kyoto University Museum, 3rd floor lecture room, Main Building

By the way, when electronic devices are used in the space environment, the die, which is a semiconductor (silicon, etc.) sealed inside the electronic device package, can be exposed to radiation, especially gamma rays containing charged particles. become.

When exposed to radiation, there are permanent damage and performance degradation due to potential fluctuations on the die, known as the total dose effect, as well as temporary malfunctions of electronic devices due to charged particles passing through the die, known as the single event effect. occurs, causing a satellite malfunction.

Therefore, the present embodiment aims to provide a radiation shielding structure for an electronic device that can reliably shield the electronic device from exposure to radiation and provide radiation resistance when the electronic device is mounted on a board. shall be.

The radiation shielding structure for an electronic device according to this embodiment includes a first shielding cover and a second shielding cover in order to shield the electronic device mounted on the wiring board from radiation. The first shielding cover is formed into a shape that covers the electronic device mounted on the surface of the wiring board using a radiation shielding material, and its entire edge is soldered to the wiring board. Further, the second shielding cover is formed into a shape that covers the back surface of the wiring board where the electronic device is mounted using a radiation shielding material, and the entire edge portion is soldered to the back surface of the wiring board. On the other hand, the wiring board is provided with a first bonding pattern area and a second bonding pattern area made of copper foil. The first bonding pattern area is formed on the front side of the wiring board in an area where the entire edge of the first shielding cover around the mounting location of the electronic device is soldered. Further, the second bonding pattern area is formed on the back side of the wiring board in an area where the entire edge of the second shielding cover is soldered.

FIG. 1 is a top view and a sectional view showing a radiation shielding structure of an electronic device according to a first embodiment. FIG. 2 is a cross-sectional view showing a radiation shielding structure of an electronic device according to a second embodiment. FIG. 3 is a top view showing the radiation shielding structure of the electronic device according to the second embodiment. FIG. 4 is a plan view showing a specific arrangement example of through holes applied to the radiation shielding structure of an electronic device according to the second embodiment. FIG. 5 is a cross-sectional view showing the state of radiation incidence when shielding is insufficient due to constraints in the structure of the first embodiment. FIG. 6 is a top view and a cross-sectional view showing a radiation shielding structure for an electronic device according to a third embodiment. FIG. 7 is a cross-sectional view of the structure of the third embodiment for explaining the adverse effects caused by a closed space. FIG. 8 is a top view and a cross-sectional view showing a radiation shielding structure for an electronic device according to a fourth embodiment. FIG. 9 is a cross-sectional view showing a radiation shielding structure of an electronic device according to a fifth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for carrying out the present invention (hereinafter simply referred to as an embodiment) will be described in detail with reference to the drawings. Note that the disclosure is merely an example, and any modifications that can be easily made by those skilled in the art while maintaining the spirit of the invention are naturally included within the scope of the present invention. In addition, in order to make the explanation clearer, the drawings may schematically represent the width, thickness, shape, etc. of each part compared to the actual aspect, but this is just an example, and the drawings are merely examples of the present invention. It does not limit interpretation. In addition, in this specification and each figure, the same reference numerals are given to components that perform the same or similar functions as those described above with respect to the existing figures, and overlapping detailed explanations may be omitted as appropriate. .

As mentioned above, when an electronic device is used in a space environment, the semiconductor die encapsulated within the package of the electronic device is exposed to radiation, particularly gamma rays containing charged particles.

When exposed to radiation, there are permanent damage and performance degradation due to potential fluctuations on the die, known as the total dose effect, as well as temporary malfunctions of electronic devices due to charged particles passing through the die, known as the single event effect. occurs, causing a satellite malfunction.

In order to improve such radiation exposure of semiconductors, conventionally, a structure has been proposed in which a shielding effect is provided in a single semiconductor device, as in Patent Document 1 and Patent Document 2. However, these methods are not compatible with all electronic devices, are expensive, and are often not commercially available.

In addition, in recent years, electronic device packaging has become known as a QFP (Quad Flat Package) package, in which external connection terminals are drawn out from all sides of the package to electrically connect the die inside the device to external circuits, power supplies, and ground. The mainstream is a BGA (Ball Grid Array) package in which external connection terminals are arranged in a grid pattern on the bottom of the package.

As devices become smaller, the spacing between the external terminals themselves has become on the order of 1 to 0.1 mm, such as 0.65 mm, making it extremely difficult to take measures to shield the device alone.

Therefore, in the following description, an embodiment in the case of a QFP (Quad Flat Package) package and an embodiment in the case of a BGA (Ball Grid Array) package will be discussed.

(First embodiment)
FIG. 1 shows a radiation shielding structure for an electronic device according to a first embodiment, in which (a) is a top view and (b) is a cross-sectional view. In FIG. 1, 11 is an electronic device in a QFP package, 12 is a wiring board on which the electronic device 11 is mounted, 131 and 132 are bonding pattern areas made of copper foil, 14 is a through hole, and 151 and 152 are shielding covers made of lead. . 1(a) shows a state where the shielding cover 151 is seen from above the mounting surface of the electronic device 11, and FIG. 1(b) shows a cross section of the mounting surface of the wiring board 12 seen from the side. ing.

As shown in FIGS. 1A and 1B, the electronic device 11 includes a die 111 housed in a package 112 made of a molding material, and a plurality of external connection terminals 113 pulled out at predetermined intervals from the four sides of the package 112. It is the composition.

The wiring board 12 has a structure in which a wiring path is formed inside, and a connection land 121 is formed at a position opposite to the terminal 113 at the mounting position of the electronic device 11, and a ground layer 122 is formed inside the board. Ru. In the wiring board 12, bonding pattern areas 131 and 132 made of copper foil (pads) are provided around the front surface of the mounting position of the electronic device 11 and the opposite back surface so as to oppose each other. Through holes 14 are formed at predetermined intervals between the copper foils in the bonding pattern regions 131 and 132. The through hole 14 is connected to a ground layer 122 formed inside the wiring board 12.

The upper shielding cover 151 has an edge facing the bonding pattern area 131 formed on the surface of the wiring board 12, covers the electronic device 11 from above, and has the edge bonded to the bonding pattern area 131. It is fixed by soldering. Further, the lower shielding cover 152 has an edge thereof facing the bonding pattern area 132 formed on the back surface of the wiring board 12, and is fixed by soldering with the edge bonded to the bonding pattern area 132. be done.

For example, in this embodiment, lead, which is easy to process, is used for the shielding covers 151 and 152 as a shielding material that shields radiation. Lead has an affinity with the solder used when mounting the electronic device 11 on the wiring board 12, and has high shielding properties by soldering.

In the wiring board 12 on which the electronic device 11 is mounted, the shielding covers 151 and 152 made of lead surround the electronic device 11 on the front surface and back surface of the board around the mounting position of the electronic device. , 152 can be directly soldered, bonding pattern regions 131 and 132 are formed using copper foil (pads).

After mounting the electronic device 11 on the wiring board 12, the shielding covers 151, 152 are firmly fixed by soldering the shielding covers 151, 152 to the bonding pattern areas 131, 132 of the wiring board 12.

As a result, the electronic device 11 is shielded from radiation from both the front side and the back side when mounted on the wiring board 12, and radiation exposure to the electronic device is reliably shielded and radiation resistance is improved. You can have it.

Further, in this embodiment, a through hole 14 connected to a ground layer 122 provided on the wiring board 11 is provided between bonding pattern areas 131 and 132 formed by copper foil (pads) for fixing the shielding covers 151 and 152. I try to form a large number of them at intervals of . According to this structure, the shielding covers 151 and 152 soldered to the bonding pattern regions 131 and 132 have the same potential as the ground layer. Therefore, a current leak path is formed, and in addition to the effect of shielding radiation, the effect of preventing charging of the electronic device 11 can be obtained.

Furthermore, heat generated in the electronic device is transferred to the shielding covers 151 and 152 via the ground layer 122 of the wiring board 12 and the through holes 14, so that a heat dissipation effect can be obtained. Furthermore, regarding the shielding covers 151 and 152, in addition to bringing them into close contact with the package 112 of the electronic device 11, the heat dissipation effect can be improved by increasing the bonding area with the bonding pattern regions 131 and 132 and the number of through holes 14. be.

(Second embodiment)
FIG. 2 is a cross-sectional view showing a radiation shielding structure of an electronic device according to the second embodiment, FIG. 3 is a top view of the structure when viewed from above in the same embodiment, and FIG. 4 is a through-hole in the same embodiment. FIG. 3 is a plan view showing an example of arrangement. 2 to 4, the same parts as in FIG. 1 are denoted by the same reference numerals, and different parts will be explained here.

In FIGS. 2 to 4, reference numeral 16 indicates a large number of through holes formed in the wiring board 12. As shown in FIG. Moreover, 171 and 172 are respectively shielding plates made of tungsten, which has a radiation shielding rate six times higher than that of lead. The shielding plate 171 is disposed between the upper surface of the package 112 of the electronic device 11 and the inner upper surface of the shielding cover 151, and faces both. Further, the shielding plate 172 is disposed between the back surface of the wiring board 12 and the inner bottom surface of the lower shielding cover 152, and is opposed to both. In this way, with the shielding plates 171 and 172 made of tungsten interposed between the electronic device 11 and the wiring board 12, the shielding covers 151 and 152 are connected to the bonding pattern area 131 of the wiring board 12, By soldering to 132, the shielding plates 171 and 172 made of tungsten, which is difficult to process, can be easily fixed, thereby achieving a further shielding effect. Note that the shielding effect can be obtained with either one of the shielding plates 171 and 172.

Further, in this embodiment, a large number of through holes 16 are formed in the wiring board 12 not only in the bonding pattern areas 131 and 132 but also in the vicinity of the electronic device in a staggered manner, and the holes of the through holes 16 are filled with solder. do. This makes it possible to reduce the amount of radiation incident from the side surface of the wiring board 12 reaching the die 111 at the radiation incident location in FIG. 2, thereby improving radiation resistance.

Specifically, in order to ensure a certain amount of attenuation effect on the incident radiation, the through holes 16 are arranged as shown in FIG. 4, for example. That is, in a lattice shape in which the sides of a plurality of hexagons are brought into close contact with each other, each apex and center point of the hexagons are used as reference points, and each through hole 16 is arranged with its midpoint aligned with the reference point. Further, the radius r of the hole diameter of each through hole 16 is set to be 1/4 or more of the side α of the hexagon. This makes it possible to pass through a path that is larger than the hole diameter without fail, and it becomes possible to guarantee a certain amount of attenuation.

In addition, from the ratio of the lead content of the solder filled in the through holes 16, for example, if the lead content is 60% and the thickness of the lead plate that can be expected to have a radiation attenuation effect is 3 mm, the total hole diameter of the through holes 16 will be doubled. The number of through holes may be increased to take 6 mm.

(Third embodiment)
FIG. 5 is a cross-sectional view showing the structure of the first embodiment, showing how radiation enters when shielding is insufficient due to constraints. FIG. 6 shows the structure of the third embodiment, which improves the radiation entering shown in FIG. The radiation shielding structure of such an electronic device is shown, in which (a) is a top view and (b) is a cross-sectional view. 6(a) shows a state where the shielding cover 151 is seen from above the mounting surface of the electronic device 11, and FIG. 6(b) shows a cross section of the mounting surface of the wiring board 12 seen from the side. ing. In FIGS. 5 and 6, the same parts as those in FIG. 1 are denoted by the same reference numerals, and different parts will be explained here.

In the first embodiment shown in FIG. 1, a large number of through holes 14 filled with solder are provided in the substrate 12 to block the incidence of radiation from the outside, but in reality, the circuit configuration is There may also be cases where it is not possible to arrange enough through holes 14 for shielding due to physical constraints such as substrate area and high pattern density for signal wiring. In this case, as shown in FIG. 5, the radiation enters the die 111 from the side surface of the wiring board 12, particularly at the point where the radiation is incident from the diagonally downward direction in the figure, on the opposite side to the surface on which the electronic device 11 is mounted. It is difficult to ensure the effectiveness of shielding radiation.

Accordingly, in the third embodiment, as shown in FIG. It is made wider than the opposing range of the shielding area by the shielding cover 151 and the bonding pattern area 131 located on the upper side in the figure. A plurality of through holes 141 are formed in the bonding pattern area 131 on the mounting surface side of the substrate 12 and are connected to lands 133 formed on the back surface side. Further, a plurality of through holes 142 are also formed in the bonding pattern region 132 on the back side of the substrate 12, and are connected to lands 134 formed on the front side. According to this structure, the shielding range by the shielding cover 152 disposed on the back surface of the wiring board 12 is wider than the shielding range by the shielding cover 151 disposed on the front surface of the wiring board 12, so that sufficient Even if a through hole cannot be arranged, it is possible to shield radiation that enters the die 111 from the obliquely lower side of the wiring board 12.

(Fourth embodiment)
FIG. 7 is a cross-sectional view of the structure of the third embodiment for explaining the adverse effects caused by the enclosed space, and FIG. 8 is an electronic device according to the fourth embodiment that improves the adverse effects caused by the enclosed space shown in FIG. FIG. 2 shows a radiation shielding structure, in which (a) is a top view and (b) is a cross-sectional view. 8(a) shows a state where the shielding cover 151 is seen from above the mounting surface of the electronic device 11, and FIG. 8(b) shows a cross section of the mounting surface of the wiring board 12 seen from the side. ing. In FIGS. 7 and 8, the same parts as those in FIG. 6 are denoted by the same reference numerals, and different parts will be explained here.

When operating electronic devices in a vacuum environment such as outer space, outgas emitted from organic materials condenses and adheres to other devices, especially optical devices, resulting in a decrease in their performance. It is known to have negative effects. To avoid this, a technique called degassing treatment (vacuum baking treatment) is used. In this method, the target device is exposed to a vacuum created in a vacuum chamber, and the gas stored inside the material used in the target device is released in advance by heating the device to a high temperature using a heater etc. It is.

As shown in FIG. 7, in the shielding structure of the third embodiment shown in FIG. 6, a shielding cover 151 and a shielding By soldering the cover 152, sealed spaces 301 and 302 are formed on the front and back sides, respectively. Depending on the material used inside the shielding cover, outgas will be generated, but since the space is sealed by the shielding, if a small amount leaks over a long period of time, the electronic device 11 inside the shielding cover will be damaged. This may have an adverse effect on other equipment. Furthermore, when exposed to an environment where atmospheric pressure changes occur, tensile stress or compressive stress is applied to the shielding cover 151, the shielding cover 152, the bonding pattern area 131, the bonding pattern area 132, and the wiring board 12, causing breakage, etc. There is a possibility that it will.

Therefore, in this embodiment, as shown in FIG. 8, a position of the wiring board 12 where the internal space 301 formed by the shielding cover 151 and the internal space 302 formed by the shielding cover 152 are connected and ventilated is provided. A hollow through hole 143 (not filled with solder) is formed in the hole to connect with the land 133 formed on the back surface, and between the internal space 302 formed by the shielding cover 152 and the external space on the front side of the wiring board 12. A hollow through hole (not filled with solder) 144 is formed at a position where the parts are connected and ventilated, and is connected to the land 134 formed on the surface. In this embodiment, the bonding pattern area 131 on the mounting surface side and the bonding pattern area 132 on the back side are each expanded inward, and through holes 143 and 144 are formed in the expanded areas. According to this structure, an outgas release path is secured between the internal space 301 formed by the shielding cover 151, the internal space 302 formed by the shielding cover 152, and the external space, as shown by the arrows in the figure, and also in an environment of pressure fluctuations. This makes it possible to prevent the generation of tensile stress or compressive stress that would otherwise occur.

(Fifth embodiment)
FIG. 9 is a cross-sectional view showing a radiation shielding structure of an electronic device according to a fifth embodiment. Here, a radiation shielding structure is shown when an electronic device 21 using a BGA package is mounted on a flexible wiring board 22. In FIG. 9, the electronic device 21 has a structure in which a die 211 is housed in a molded resin of a BGA package 212, and solder balls serving as external connection terminals 213 are arranged in a grid pattern on the bottom surface of the package 212.

In this embodiment, as a means for reliably shielding radiation incident on the entire periphery of the electronic device 21, as shown in FIG. Use the board. The electronic device mounting portion of the wiring board 22 is provided with a land 221 for connecting the external connection terminal 213 of the electronic device 21 to the internal wiring path.

As the radiation shielding structure, a box-shaped shielding cover 231 with one side open and made of lead that shields radiation is used to cover the flexible wiring board 22 on which the electronic device 21 is mounted, within the mounting range of the electronic device 21. The electronic device 21 is accommodated from above, including the bent side surface of the wiring board 22, with the outer side bent into a mountain fold. In addition, a box-shaped shielding cover is provided using a convex-shaped shielding block 232 so that a gap narrower than the opening surface of the shielding cover 231 and allowing the wiring board 22 to be pulled out is secured by the lead shielding radiation. The wiring board 22 housed in 231 is pressed from the back side.

The wiring board 22 is bent into a valley fold along the edge of the shielding cover 231. At this time, copper foils 241 and 242 are provided in the joining area of the wiring board 22 and the shielding cover 231, and the shielding cover 231 is fixed by soldering with its edges joined to the copper foils 241 and 242. .

Further, copper foils 251 and 252 are provided at the joint portions of the wiring board 22 and the shielding block 232, and the shielding block 232 is fixed to the copper foils 251 and 252 by soldering.

All of the copper foils 241, 242, 251, and 252 are preferably connected to the ground of the wiring board 22. As a result, by soldering the shielding cover 231 and the shielding block 232 to the ground, it is possible to obtain a heat dissipation effect as well as an effect of preventing the electronic device 21 and the wiring board 22 from being charged.

That is, in this embodiment, the shielding cover is formed in a box shape with one side open and designed to have a depth that can accommodate the sides of the electronic device 21 and the flexible wiring board 22 on which the electronic device 21 is mounted. 231 from the upper surface of the electronic device 21, and for the lower surface of the electronic device 21, for example, a convex lead made of lead is sized to fit with the shielding cover 231 on the upper surface with a gap that allows the flexible wiring board 22 to be pulled out. Using the shielding block 232, the part of the wiring board 22 where the electronic device 21 is mounted is sandwiched between the box-shaped shielding cover 231 and the convex-shaped shielding block 232.

In this way, according to the structure of this embodiment, it is possible to reliably block radiation from entering the die 211 from all around, including radiation from diagonally below the end of the wiring board 22 .

Furthermore, since the elastic flexible wiring board 22 is sandwiched between the shielding cover 231 and the shielding block 232, tensile stress or compressive stress due to atmospheric pressure fluctuations can be absorbed by the elasticity of the flexible board and each shielding cover 231, It has a structure that is buffered by a gap of 232, and also has the effect of securing a release path for outgas.

In the above embodiment, several shielding structures are used together, but they may be used individually. In addition, the present invention is not limited to the above-mentioned embodiments as they are, and in the implementation stage, the constituent elements can be modified and embodied without departing from the spirit of the invention. Moreover, various inventions can be formed by appropriately combining the plurality of components disclosed in each of the above embodiments. For example, some components may be deleted from all the components shown in the embodiments. Furthermore, components of different embodiments may be combined as appropriate.

DESCRIPTION OF SYMBOLS 11... Electronic device, 111... Die, 112... Package, 113... External connection terminal, 12... Wiring board, 121... Land, 122... Ground layer, 131, 132, 133, 134... Bonding pattern area (copper foil), 14 , 141, 142, 143, 144... Through hole, 151, 152... Shielding cover (lead), 16... Through hole, 171, 172... Shielding plate (tungsten), 21... Electronic device, 211... Die, 212... Package, 213... External connection terminal (solder ball), 22... Flexible wiring board, 221... Land, 231... Shielding cover (lead), 232... Shielding block (lead), 241, 242, 251, 252... Copper foil, 301, 302 ...Inner space.

Claims (9)

A radiation shielding structure for an electronic device that shields an electronic device mounted on a wiring board from radiation,
a first shield formed in a shape to cover the electronic device mounted on the surface of the wiring board by the radiation shielding material, and whose entire edge is soldered to the wiring board;
a second shield formed into a shape that covers the back surface of the mounting location of the electronic device of the wiring board with the radiation shielding material, and whose entire edge is soldered to the back surface of the wiring board;
A first bonding pattern area made of copper foil is formed on the front side of the wiring board in a region where the entire edge of the first shield of the wiring board is soldered around the mounting location of the electronic device. and,
A radiation shielding structure for an electronic device, comprising: a second bonding pattern region made of copper foil formed in a region where the entire edge of the second shield is soldered on the back side of the wiring board. 2. The radiation shielding structure for an electronic device according to claim 1, wherein the first bonding pattern area and the second bonding pattern area are both connected to the ground of the wiring board. The wiring board includes a plurality of through holes that penetrate and connect the first bonding pattern area and the second bonding pattern area, and heat generated in the electronic device is transferred to the first bonding pattern area through the plurality of through holes. The radiation shielding structure for an electronic device according to claim 1, wherein the radiation is transmitted to the first shielding body and the second shielding body. The first shield is interposed between the electronic device and the first shield, or between the surface of the wiring board facing the mounting position of the electronic device and the second shield, and 2. The radiation shielding structure for an electronic device according to claim 1, comprising: a shielding body and a shielding plate made of a shielding material having a higher shielding rate than the second shielding body. 2. The radiation shielding for an electronic device according to claim 1, wherein a plurality of through holes are densely arranged in the vicinity of a mounting position of the electronic device on the wiring board, and each hole of the plurality of through holes is filled with solder. structure. A lattice shape in which the sides of a plurality of hexagons are brought into close contact with each other, each vertex of the plurality of hexagons and the center point of the plurality of hexagons are used as reference points, and the plurality of through holes are set with the middle point as the reference point. The radius r of each of the plurality of through holes is set to be 1/4 or more of the side α of the hexagon so that a certain amount of radiation is absorbed by the incident radiation. The radiation shielding structure for an electronic device according to claim 5 , which ensures an attenuation effect. The electronic device according to claim 1, wherein the shielding range by the second shielding body disposed on the back surface of the wiring board is wider than the shielding range by the first shielding body disposed on the front surface of the wiring board. Radiation shielding structure of the device. A first internal space formed by the first shield on the front side of the wiring board and a second internal space formed by the second shield on the back side of the wiring board are connected. Hollow through holes are formed in the wiring board at a position where the second internal space and an external space on the front side of the wiring board are connected to create a ventilation state, respectively. 8. The radiation shielding structure for an electronic device according to claim 7. The wiring board is made of a flexible material,
The first shielding body is formed into a box shape with one side open by a shielding material that shields the radiation, and the first shielding body is configured to cover the wiring board on which the electronic device is mounted so that the outside of the mounting area of the electronic device is mounted on a mountain. In a folded state, the electronic device is accommodated from above including the bent side surface of the wiring board,
The second shield is formed into a convex shape narrower than the opening surface of the first shield by a shielding material that shields the radiation, and the second shield has a convex shape that is narrower than the opening surface of the first shield, and the wiring board accommodated in the first shield is exposed to the wiring board from the back side. The radiation shielding structure for an electronic device according to claim 1, wherein the convex upper surface is housed inside the first shielding body.

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